Differential push-pull (DPP) method has been known as one of the tracking servo systems used in a drive apparatus for an optical recording disc such as CD-R and CD-RW.
According to the DPP method, calculation based on output signals from each photoreceptor, which are respectively obtained from the main beam and two side beams, allows a tracking error signal to occur.
Particularly, a beam emitted by a laser diode (light source) 1 forms three beam spots consisting of a spot generated by zeroth-order diffraction light (hereinafter referred to as “main beam 21”) and two spots generated by first-order diffraction light (hereinafter referred to as “side beams 22A, 22B”) on an optical disc 6 through diffracting means (a diffraction grating) 2, collimator lens 4, and objective lens 5, which are set on a forward course of the emitted beam as shown in FIG. 1.
A center 36 (hereinafter referred to as a “centerline of track”) of each track 26 in the optical disc 6 rotationally driven is irradiated with the main beam 21 as illustrated in an illustration encircled by dotted lines shown from A direction in FIG. 1. A disc surface between the track and an advanced adjacent track is irradiated with the side beam 22A, which defines it as an advanced beam. Another disc surface between the track and a subsequent adjacent track is irradiated with the side beam 22B, which defines it as a subsequent beam. Respective photoreceptors of photo-detector 8 receive reflected lights for these beams (according to a theory of tracking).
The photo-detector 8 comprises a main photo-detector (MPD) for receiving a main spot by the main beam 21, side photo-detector (SPD1) for receiving a side spot from the side beam 22A, and side photo-detector (SPD2) for receiving a side spot from the side beam 22B, as illustrated in an illustration encircled by dotted lines shown from B direction in FIG. 1.
In the photo-detector 8, the main photo-detector (MPD) is divided into 4 pieces of elements vertically and horizontally each receiving the main spot by the main beam 21. Respective side photo-detectors SPD1, SPD2 are divided into 2 pieces of elements vertically and the side photo-detector SPD1 receives the side spot by the side beam 22A as well as the side photo-detector SPD2 receives the side spot by the side beam 22B. Incidentally, referring to output signals from the respective divided elements as A, B, C, D, E, F, G, and H, respectively, the calculation based on these signals allows the tracking servo error signal to occur.
The main push-pull signal (MPP) occurs based on the output signal from the MPD. Side push-pull signals (SPP1 and SPP2) occur based on the output signals from the respective SPD1 and SPD2. DPP signals are obtained according to a following operational expression:MPP=(A+D)−(B+C)SPP=SPP1+SPP2=(F−E)+(H−G)
                    DPP        =                ⁢                  MPP          -                      K            *            SPP                                                  =                ⁢                              (                          A              +              D                        )                    -                      (                          B              +              C                        )                    -                      K            ⁢                          {                                                (                                      F                    -                    E                                    )                                +                                  (                                      H                    -                    G                                    )                                            }                                          where K is a constant set for adjusting a light intensity of the diffraction light of zeroth-order, and plus and minus first-order.
The DPP method has such a drawback that offset occurs in the SPP signal when the side beams 22A, 22b are not positioned at their center of the SPD1, SPD2 and others. If so, two signals SPP1, SPP2 have almost the same amplitude, thereby allowing cancellation of the offset by the addition of these signals at the reproduction. At the recording, however, they have different amplitudes, thereby failing to cancel the offset. This causes the tracking servo error signal including the offset, resulting in some inconvenience that the tracking is unstable, at the worst case of de-tracking thereof, and the less recording performance is delivered.